Current-driven writing process in antiferromagnetic Mn2Au for memory applications

Current pulse driven Néel vector rotation in metallic antiferromagnets is one of the most promising concepts in antiferromagnetic spintronics. We show microscopically that the Néel vector of epitaxial thin films of the prototypical compound Mn2Au can be reoriented reversibly in the complete area of cross shaped device structures using single current pulses. The resulting domain pattern with aligned staggered magnetization is long term stable enabling memory applications. We achieve this switching with low heating of ≈20 K, which is promising regarding fast and efficient devices without the need for thermal activation. Current polarity dependent reversible domain wall motion demonstrates a Néel spin-orbit torque acting on the domain walls.


SUPPLEMENTARY INFORMATION
Current pulse induced sample heating All current pulsing experiments are associated with ohmic heating of the sample. As Mn 2 Au is a metal with an almost linear resistance versus temperature relation, the current pulse induced heating can be measured via a resistance measurement during the pulse as described in the Methods section. Supplementary Fig. S1 shows the relative resistance increase of a Mn 2 Au(001) sample patterned in a cross structure with a central section of 10 µm × 10 µm as in Fig. 1 of the main text, but with additional leads to enable 4-probe resistance measurements. Different pulse lengths and amplitudes result in different heating. The dashed vertical lines in the main figure indicate the required current densities for complete switching of the Néel vector. From comparison with the extrapolated temperature dependent resistance measurement shown in the inset, the indicated temperatures associated with the switching using different pulse lengths are shown in the main panel.
Probing Néel vector reorientation by transverse resistance measurements Electrical detection of current pulse induced Néel vector alignment is also possible via measurements of the transverse resistance or planar Hall-Effect. Considering the rather small electrical signal, this is more easy to measure than the longitudinal AMR, as there is no offset voltage. In Supplementary Fig. S2, the measurement of the transverse resistance changes is shown in analogy to Fig. 4 of the main text.
Stability of the Néel vector alignment All images of current induced Néel vector reorientation shown in the main text were obtained at MAXPEEM (MAX IV). Four months later at Diamond Light Source, we were able to image the sample shown in Fig. 2 of the main text again. As shown in Supplementary Fig. S3, we observed exactly the same domain pattern again, i. e. the current induced Néel vector oriented domain configuration obtained four month ago was completely preserved.
Additionally, we demonstrate the absence of relaxation on short time scales, i. e. seconds, by measurements of the transverse resistance after a sequence of current pulse in-FIG. 1. S1: Current pulse induced sample heating. The measurements in the main panel show the maximum sample resistance (normalized to the 300 K resistance) during application of current pulses with different length and amplitude. The dashed vertical lines show the required currents for full Néel vector reorientation corresponding to Fig. 1. The inset shows the temperature dependence of the normalized resistance obtained from a standard dc measurement. Its extrapolation (red dashed line) is used to convert the resistance increase during current pulsing into a temperature increases ∆T shown in the graph. duced reversible switching. The data shown in Supplementary Fig. S4 was obtained in the same way as for Supplementary Fig. S2 (here with J pulse = 2.9 × 10 11 A/m 2 ), but we kept measuring the resistance after the last switching current pulse for 1.5 h. The raw data shown was obtained at a sampling rate of 2 data points per second. Also on this time scale, no relaxation of the resistance after the last switching pulse was observed.
Microstructure of Mn 2 Au(001) thin films All epitaxial Mn 2 Au thin films used for this manuscript were prepared by sputtering as described in the Methods sections. Beyond their characterization shown in ref.
[20], we here want to emphasize their long range crystallographic order by showing in Supplementary Fig. S5 scanning transmission electron microscopy images. Panel a with the highest magnifications shows the Mn 2 Au lattice with atomic resolution (on top of this sample is a Permal-  Fig. 4, panel b. The transverse resistance was measured after each 1 ms current pulse with J pulse = 3 × 10 11 A/m 2 applied alternately in perpendicular directions as indicated by the blue and green arrows in Fig. 4, panel b. The color of the data points corresponds to the arrows indicating the current direction. loy layer, which was not present in the other samples discussed here.). Panel b with a larger field of view shows a MnAu impurity phase at the interface with the Ta buffer layer, which, however, does not disturb the growth of the Mn 2 Au layer on top of it. Panel c shows the largest field of view demonstrating the long range coherence of the epitaxial growth.
For obtaining the STEM images, an electron trans-parent cross-section lamella was prepared using a 30 kV focused Ga+ ion beam and scanning electron microscope (FIB-SEM) FEI Helios platform. The ion beam energy was decreased to 5 kV for the final thinning steps. Scanning transmission electron microscopy (STEM) was carried out using an FEI Titan TEM equipped with a Schottky field emission gun operated at 200 kV, a CEOS probe aberration corrector and a high angle annular dark-field FIG. 4. S4: Second time-scale stability of switched state. Raw data (black data points) and averaged data points (blue and green data points as in Supplementary Fig. S2) over 200 raw data points (blue and green data points as in Supplementary Fig. S2) of the transverse resistance of a Mn2Au thin film. Here, the transverse resistance was measured after each 1 ms current pulse with J pulse = 2.9 × 10 11 A/m 2 applied alternately in perpendicular directions followed by pure resistance measurements without pulsing. No relaxation was observed.